This invention relates to the field of optoelectronic devices, and more particularly to resonant reflectors for use with optoelectronic devices.
Various forms of optoelectronic devices have been developed and have found widespread use including, for example, semiconductor photodiodes, semiconductor photo detectors, etc. Semiconductor lasers have found widespread use in modem technology as the light source of choice for various devices, e.g., communication systems, compact disc players, and so on. For many of these applications, a semiconductor laser is coupled to a semiconductor detector (e.g., photodiode) through a fiber optic link or even free space. This configuration provides a high-speed communication path, which, for many applications, can be extremely beneficial.
A typical edge-emitting semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers often called cladding layers. The low bandgap layer is termed the xe2x80x9cactive layerxe2x80x9d, and the cladding layers serve to confine both charge carriers and optical energy in the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. When current is passed through the structure, electrons and holes combine in the active layer to generate light.
Another type of semiconductor laser is a surface emitting laser. Several types of surface emitting lasers have been developed including Vertical Cavity Surface Emitting Lasers (VCSEL). (See, for example, xe2x80x9cSurface-emitting microlasers for photonic switching and interchip connectionsxe2x80x9d, Optical Engineering, 29, pp.210-214, March 1990, for a description of this laser). For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled xe2x80x9cTop-emitting Surface Emitting Laser Structuresxe2x80x9d, which is hereby incorporated by reference, and U.S. Pat. No. 5,475,701, issued on Dec. 12, 1995 to Mary K. Hibbs-Brenner, and entitled xe2x80x9cIntegrated Laser Power Monitorxe2x80x9d, which is hereby incorporated by reference. Also, see xe2x80x9cTop-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 xcexcmxe2x80x9d, Electronics Letters, 26, pp. 710-711, May 24, 1990.)
Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry, including their amenability to one- and two-dimensional arrays, wafer-level qualification, and desirable beam characteristics, typically circularly symmetric low-divergence beams.
VCSELs typically have an active region having bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks, often formed by interleaved semiconductor layers each a quarter wavelength thick at the desired operating wavelength (in the medium). The mirror stacks are typically of opposite conductivity type on either side of the active region, and the laser is typically turned on and off by varying the current through the mirror stacks and the active region.
High-yield, high performance VCSELs have been demonstrated and exploited in commercialization. Top-surface-emitting AlGaAs-based VCSELs are producible in a manner analogous to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields. VCSELs are expected to provide a performance and cost advantage in fast (e.g., Gbits/s) medium distance (e.g., up to approximately 1000 meters) single or multi-channel data link applications, and numerous optical and/or imaging applications. This results from their inherent geometry, which provides potential low-cost high performance transmitters with flexible and desirable characteristics.
A related photodetector is known as a resonant cavity photo detector (RCPD). Resonant cavity photodetectors are typically constructed similar to VCSELs, but operate in a reverse bias mode. A resonant cavity photodetector may be more efficient than a standard photodiode because the light that enters the optical cavity, through one of the mirrors, may be effectively reflected through the active region many times. The light may thus be reflected between the mirror stacks until the light is either absorbed by the active region or until it escapes through one of the mirror stacks. Because the mirror stacks are typically highly reflective near resonance, most of the light that enters the cavity is absorbed by the active region.
For many optoelectronic devices that have a resonant cavity, the top and/or bottom mirror stacks are Distributed Bragg Reflector (DBR) mirrors. DBR mirrors AlGaAs and AlAs. Often, both the top and bottom mirror stacks include a significant number of DBR mirror periods to achieve the desired reflectance. One way to reduce the number of DBR mirror periods that are required is to replace some of the DBR mirror periods with a resonant reflector. Such a configuration is disclosed in U.S. Pat. No. 6,055,262, entitled xe2x80x9cResonant Reflector For Improved Optoelectronic Device Performance And Enhanced Applicabilityxe2x80x9d, which is incorporated herein by reference. A typical resonant reflector may include, among other things, a waveguide and a grating.
Despite the advantages of using a resonant reflector in conjunction with a DBR mirror stack, it has been found that the reflectivity of the resonant reflector can be limited if it is not properly isolated from adjacent conductive layers. Too much energy in the guided-mode in the waveguide overlaps into the lossy, conductive DBR films of the optoelectronic device. What would be desirable, therefore, is an optoelectronic device that provides isolation between the resonant reflector and adjacent conducting layers of the optoelectronic device.
The present invention overcomes many of the disadvantages of the prior art by providing an optoelectronic device that provides isolation between a resonant reflector and an adjacent conducting layer of the optoelectronic device. Isolation is preferably accomplished by providing a dielectric buffer or cladding layer between the resonant reflector and the adjacent conducting layer of the optoelectronic device. The cladding or buffer layer is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide of the resonant reflector, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the adjacent conductive layer of the optoelectronic device.
In one illustrative embodiment of the present invention, an optoelectronic device includes a top mirror and a bottom mirror, with an active region situated therebetween. The top mirror and bottom mirror are Distributed Bragg Reflector (DBR) mirrors made from alternating layers of semiconductor materials that are doped to be at least partially conductive. Current can be provided through the active region and DBR mirrors to activate the device.
A resonant reflector is positioned adjacent a selected one of the top or bottom mirrors of the optoelectronic device. The resonant reflector preferably has a waveguide and a grating. The waveguide and grating are preferably configured such that a first-diffraction order wave vector of the grating substantially matches the propagating mode of the waveguide. A cladding or buffer layer is positioned between the resonant reflector and the selected top or bottom mirror. The cladding or buffer layer is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the selected top or bottom mirror.
An illustrative method for forming such an optoelectronic device includes providing a bottom mirror on a substrate. The bottom mirror is preferably a DBR mirror stack, and is doped to be at least partially conductive. An active region is then formed on the bottom mirror, followed by a top mirror. Like the bottom mirror, the top mirror is preferably a DBR mirror stack, and is doped to be the opposite conductivity type of the bottom mirror. If desired, the active region may include cladding layers on either side to help focus the light and current in the active region. Next, a deep H+ ion implant may be provided to provide a gain guide aperture, as is known in the art. While a deep H+ implant is provided as an illustration, it is contemplated that any type of current and field confinement may be used, including for example, gain-guided, oxide-confinement, or any other means. Finally, contacts may be provided on the top mirror and on the bottom surface of the substrate to provide electrical contact to the optoelectronic device.
Next, a cladding or buffer layer is provided above the top mirror. A resonant reflector is then provided adjacent the cladding or buffer layer. The resonant reflector preferably includes a waveguide and a grating which are configured such that a first-diffraction order wave vector of the grating substantially matches a propagating mode of the waveguide. To isolate the resonant reflector from the optoelectronic device, and in particular the conductive top mirror, the cladding or buffer layer is preferably sufficiently thick to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the top mirror. Alternatively, or in addition, the cladding or buffer layer may be formed from a material that has a sufficiently low refractive index relative to the refractive index of the waveguide to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the top mirror. The cladding or buffer layer is preferably non-conductive.
In another illustrative method, a resonant reflector is formed on a first substrate, and at least part of an optoelectronic device is formed on a second substrate. The first substrate is then bonded to the second substrate to complete the device. More specifically, a first substrate having a front side and a backside is provided. A resonant reflector is formed on the front side of the second substrate. Then, a second substrate having a front side and a backside is provided. At least part of an optoelectronic device is formed on the front side of the second substrate. The optoelectronic device may include, for example, a bottom mirror, an active region and a top mirror, as discussed above. Thereafter, the front side of the first substrate is bonded to the front side of the second substrate to complete the optoelectronic device.
The first substrate may be bonded to the second substrate using an optical epoxy that is sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide of the resonant reflector, so that energy from the evanescent wave vector is substantially prevented from entering the optoelectronic device on the first substrate. For top emitting devices, a collimating microlens may be provided on the backside of the first substrate. For back emitting devices, a collimating microlens may be provided on the backside of the second substrate. In either case, the collimating microlens is preferably in registration with the output of the optoelectronic device.
The present invention also contemplates forming a number of optoelectronic devices on a common substrate. One application for such a configuration is a monolithic transceiver that includes one or more light emitting devices and one or more light receiving devices. Both the light emitting and light receiving devices are preferably formed on a common substrate. In one example, a bottom mirror is first formed on the common substrate. The bottom mirror may serve as the bottom mirror for more than one of the optoelectronic devices, and is preferably a DBR mirror stack that is doped to be at least partially conductive. An active region is then formed on the bottom mirror, followed by a top mirror. Like the bottom mirror, the top mirror is preferably a DBR mirror stack, and is doped to be the opposite conductivity type of the bottom mirror. Contacts may be provided on the top mirror and on the bottom surface of the substrate to provide electrical contact to each of the optoelectronic devices.
Next, a cladding or buffer layer may be provided above the top mirror. A resonant reflector may then be provided on the cladding or buffer layer. The resonant reflector may include a waveguide and a grating film. For some optoelectronic devices, such as top emitting devices, the grating film may be etched to form a grating. This may substantially increase the reflectivity of the resonant reflector in those regions. For other optoelectronic devices, such as top receiving devices, the grating film may be etched to include a different grating structure (e.g., wider spectral bandwidth) or remain non-etched which reduces the reflectivity of the resonant reflector thereby allowing light to more easily enter the optical cavity. For yet other optoelectronic devices, such as Metal-Semiconductor-Metal (MSM) receiving devices, the grating film may be removed altogether, and a metal grid may be provided on the cladding or buffer layer.
In another illustrative embodiment of the present invention, an improved resonant reflector and method of making the same is provided. In this embodiment, a waveguide and a grating film are both provided. The grating film is etched to form two or more spaced grating regions separated by one or more spaced etched regions. Rather than varying the fill factor of the grating to achieve the desired optical properties of the resonant reflector, this embodiment contemplates controlling the etch depth of the spaced etched regions. It has been found that this may increase the producibility and yield of the resonant reflector.